The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community and is the subject of numerous patents.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient driving force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, and hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatic, unsaturated, and saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, e.g., naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons, e.g., aromatic and/or olefinic from gasoline-boiling-range mixtures by the selective permeation of the aromatic through certain nonporous cellulose ester membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatic from saturates via pervaporation. U.S. Pat. Nos. 5,028,685 and 5,093,003 disclose halogenated polyurethanes and membranes therefrom for separating aromatics from saturates.
U.S. Pat. Nos. 4,944,880 and 4,990,275 describe polyimide/aliphatic polyester copolymers and membranes therefrom for the separation of aromatics from saturates. U.S. Pat. Nos. 4,946,594 and 4,997,906 describe crosslinked copolymers of aliphatic polyester diols and dianhydrides and membranes therefrom for the separation of aromatics from saturates.
U.S. Pat. No. 4,976,868 covers the use of polyester membranes (e.g., polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate/cyclohexane-dimethanol terephthalate) for aromatics/saturates separation. U.S. Pat. Nos. 5,012,036, 5,012,035, and 5,019,666 teach the use of polyarylate, polyphthalate-carbonate, and nonporous polycarbonate membranes, respectively, to separate aromatics from saturates. U.S. Pat. No. 5,055,631 discloses sulfonated polysulfone membranes for the separation of aromatics from saturates. U.S. Pat. No. 5,128,439 describes saturated polyesters and crosslinked membranes therefrom for aromatics/saturates separation.
The present invention describes new polyimide/aliphatic polyester copolymers. Compared to those described in U.S. Pat. Nos. 4,944,880 and 4,990,275, the copolymers of the present invention do not contain pendent carboxylic acid groups. In addition, the present invention describes a synthesis of the copolymers by reaction of trimellitic anhydride acid chloride, a polyester diol, and an aromatic diamine. Furthermore, the present invention describes the use of the above copolymers to make membranes for the separation of aromatics from saturates.
Compared to distillation, membrane permeation can lead to considerable energy savings. A membrane can separate a mixture of aromatics and saturates, e.g., a heavy catalytic naphtha, into a high-octane, mainly aromatic permeate and a high-cetane, mainly saturated retentate. Both permeate and retentate are more valuable than the starting heavy catalytic naphtha.